diff --git a/Services/BackupEngine.swift b/Services/BackupEngine.swift
index 5902b00..26900b0 100644
--- a/Services/BackupEngine.swift
+++ b/Services/BackupEngine.swift
@@ -130,15 +130,47 @@ final class BackupEngine: ObservableObject {
         signposter.endInterval("ManifestLoad", spManifest, "\(baseIndex.totalCount) entries")
         logger.info("Manifest base: \(baseIndex.totalCount) entries, isFilenameOnly=\(baseIndex.isFilenameOnly)")
 
+        // ── 3b. Build filter index ─────────────────────────────────────────
+        // When the NAS directory has more files than Kisani's manifest (e.g. files
+        // uploaded by other tools), a manifest-only index will misidentify those files
+        // as "not backed up" and re-upload them on every "Back up again" tap.
+        // Use the broader NAS directory listing as the filter when the manifest is sparse.
+        let filterIndex: ManifestIndex
+        let cachedDirCount = await NASManifestCache.shared.directoryCount ?? 0
+        if baseIndex.totalCount < cachedDirCount {
+            if let cachedFilenames = await NASManifestCache.shared.directoryFilenames,
+               cachedFilenames.count > baseIndex.totalCount {
+                filterIndex = await Task.detached(priority: .utility) {
+                    ManifestIndex(nasFilenames: cachedFilenames)
+                }.value
+                logger.info("Filter index: \(cachedFilenames.count) cached NAS filenames (manifest=\(baseIndex.totalCount))")
+            } else {
+                // No cached filenames — do a fresh listing (connection is already open)
+                let nasItems = (try? await transfer.listDirectory(at: connection.remotePath)) ?? []
+                let names = nasItems.compactMap { item -> String? in
+                    guard !item.isDirectory, !item.name.hasPrefix("."),
+                          item.name != BackupManifest.remoteFilename else { return nil }
+                    return item.name
+                }
+                await NASManifestCache.shared.setDirectoryFilenames(names)
+                filterIndex = await Task.detached(priority: .utility) {
+                    ManifestIndex(nasFilenames: names)
+                }.value
+                logger.info("Filter index: \(names.count) fresh NAS filenames (manifest=\(baseIndex.totalCount))")
+            }
+        } else {
+            filterIndex = baseIndex
+        }
+
         try Task.checkCancellation()
 
         // ── 4. Build pending queue off the main actor ──────────────────────
         let spFilter = signposter.beginInterval("BuildPendingQueue")
         let pendingAssets: [PhotoAsset] = await Task.detached(priority: .userInitiated) {
             assets.filter { asset in
-                !baseIndex.matches(localIdentifier: asset.localIdentifier) &&
-                !(baseIndex.isFilenameOnly && baseIndex.matches(filename: asset.filename)) &&
-                !baseIndex.matchesUnclaimed(filename: asset.filename)
+                !filterIndex.matches(localIdentifier: asset.localIdentifier) &&
+                !(filterIndex.isFilenameOnly && filterIndex.matches(filename: asset.filename)) &&
+                !filterIndex.matchesUnclaimed(filename: asset.filename)
             }
         }.value
         signposter.endInterval("BuildPendingQueue", spFilter, "\(pendingAssets.count) pending")